High temperature diffusion furnaces are well known to the semiconductor industry. Heat treatment in high temperature diffusion furnaces is a part of the manufacturing process for silicon wafers whereby, for example, doping elements such as boron can be introduced into the molecular structure of the semiconductor material. Heating cycles for the furnaces must be controlled accurately with respect to time and temperature. There is also a requirement that the diffusion furnace be made durable enough to withstand repeated heating and cooling cycles. Further, for purposes of the manufacturing processes, it is important that the diffusion furnace quickly reach the desired temperature, maintain the temperature for a preselected period of time and then quickly reduce the temperature to the desired level.